In the manufacture of metal containers, such as beverage containers, metal blanks are stamped and then formed by drawing and ironing, drawing and redrawing or other forming processes the blanks into a desired container conformation and finally decorated. During the forming process, lubricants and coolants are deposited on the metal surfaces. Residual quantities of lubricants and coolants must be removed from the surface of the metal prior to painting, coating, decorating or packaging the finished metal product.
Currently, the industry uses cleaning agents consisting of either relatively strong acid or caustic solutions. Primarily, strong acid solutions consisting of hydrofluoric, sulfuric, phosphoric, nitric, and similar acids are used. For example, U.S. Pat. No. Re. 32,661 to Binns discloses a method of cleaning aluminum using an acidic aqueous cleaning solution having a pH less than 2. Similarly, U.S. Pat. No. 3,969,135 to King discloses a composition and process for cleaning aluminum using a low temperature aqueous acidic solution with a pH of less than 2. Typically, acid or caustic cleaning solutions are rinsed from containers with water.
While the use of acidic and caustic solutions to clean metal is effective to remove oils, such use presents multiple problems in the treatment of used cleaning and rinse solutions, as well as negative effects on the appearance and integrity of the containers. Specifically, used acid or caustic cleaning agents require neutralization prior to their discharge. Several disadvantages are encountered in such neutralization, including the cost of the materials and the handling problems associated with such toxic chemicals. The corrosive nature of acid and caustic cleaning solutions also damages processing equipment used in the cleaning of metal containers.
Methods for cleaning metal containers with acidic or caustic solutions also makes the recycling of the cleaning agent, lubricants, coolants and aqueous factions of the process extremely difficult. Once mixed with rinse water, acidic and caustic cleaning agents are not readily recoverable. Therefore, as discussed above, such mixtures must be treated in a manner that permits their ecological disposal or release. The prior art does not teach an economical method of treating conventional cleaning agents, lubricants, coolants or water used in typical processes.
Further, the acidic cleaning agents currently in use etch the metal surfaces of containers being cleaned, creating grooves and gouges in the metal surface and thereby, producing waste solutions with dissolved metal fines. These metal fines create waste disposal problems.
Acidic etching can also damage containers by creating or enlarging pin-sized holes or gouges in the containers to make "pinhole leakers", making such containers unusable. For example, during drawing and ironing procedures, manganese-iron particle complexes in aluminum can create such holes or gouges. Acidic cleaning agents deepen such holes and gouges, which in turn reduces the reflectivity of the metal surface and creates problems in the adherence of paint to the metal surface. The prior art also teaches the use of active fluoride solutions to assist in the dissolution of aluminum fines from metal surfaces. Fluoride components accelerate attack upon metal surfaces in order to remove metallic particles resulting from the forming operation. Excessive fluoride levels are known to result in the etching of metallic surfaces and contribute to the disposal problems encountered by container manufacturers.
In view of the above, a need exists for a new method for manufacturing metal containers. The present invention addresses this need by providing a method for manufacturing metal containers using a cleaning agent having a substantially neutral pH capable of removing lubricants and coolants and by providing an economical method of recycling the cleaning agent, lubricants, coolants and water used in the process, without the problems associated with conventional acidic and caustic cleaning solutions.